ClickHouseIO: Add Decimal(P, S) support for fixed-point columns

[tomnewton]

ClickHouseIO's TableSchema and column-type parser had no notion of ClickHouse's fixed-point Decimal family, so ClickHouseIO.getTableSchema() failed on the DESCRIBE TABLE output of any table containing a Decimal column and pipelines carrying monetary/fixed-point values could not write to ClickHouse at all.

This change adds first-class Decimal(P, S) support to ClickHouseIO:

• Schema model — new TypeName.DECIMAL; ColumnType carries scale alongside the existing precision (validated: P in [1, 76], S in [0, P]), with a ColumnType.decimal(precision, scale) factory. Bare Decimal is Decimal(10, 0) and Decimal(P) is Decimal(P, 0), matching ClickHouse's defaults.
• Parser — JavaCC grammar rules for Decimal[(P[, S])] and the width aliases Decimal32(S) / Decimal64(S) / Decimal128(S) / Decimal256(S) (pinned to precisions 9/18/38/76 per ClickHouse's synonym rule; DESCRIBE TABLE canonicalizes the aliases to Decimal(P, S) anyway). Reachable through Nullable(...) and Array(...) via the existing primitive() rule. Lexical, syntactic and range-validation failures all surface as the parser's uniform failed to parse error.
• Beam field-type mapping — Decimal(P, S) maps to FixedPrecisionNumeric.of(P, S) (base type DECIMAL, values are BigDecimal), preserving the declared precision/scale through getEquivalentSchema and giving an early, loud failure at Row construction for values whose digits genuinely exceed the declared precision. This is the same logical type JdbcIO uses for NUMERIC.
• Writer — serializes the unscaled value as a little-endian signed integer whose width (32/64/128/256 bits) is chosen from the declared precision, delegating to the ClickHouse client's BinaryStreamUtils.writeDecimal. Fractional digits beyond the column scale are truncated toward zero, consistent with ClickHouse's own "excessive digits in a fraction are discarded (not rounded)" semantics. The truncated value is then bounded by the declared precision and rejected with an IllegalArgumentException if out of range: writeDecimal alone only range-checks against the backing storage width, which is wider than the declared type, and ClickHouse's RowBinary reader does not re-check — so without this a value like 100000 would be stored in a Decimal(5, 0) column whose declared maximum is 99999.
• Defaults — ColumnType.parseDefaultExpression handles Decimal literals (DEFAULT 1.23), so getTableSchema no longer throws for tables with defaulted Decimal columns, and writeRow's existing null-substitution works for them.

Tests:

• TableSchemaTest — parser cases for Decimal(10, 2), Decimal(5), bare Decimal, all four width aliases, Nullable(Decimal(10, 2)), Array(Decimal(38, 10)); uniform failed to parse errors for Decimal(77, 2), Decimal(0), Decimal(9, 10), Decimal(9, -1) and Decimal(abc); factory range validation; schema-mapping tests including nullable; a test pinning that Row construction under the mapped FixedPrecisionNumeric type rejects values exceeding the declared precision (the layer that guards the declared range, since the writer checks only the storage width); parseDefaultExpression for positive and negative literals.
• ClickHouseWriterTest — byte-level tests locking the wire format for every width bucket: little-endian unscaled Int32, scaling below the column scale, 16 × 0xFF two's-complement sign extension for a negative Decimal128, a 32-byte Decimal256 encoding, truncation-toward-zero of excess fractional digits, storage-width overflow rejection, and null/non-null through writeNullableValue. Declared-precision enforcement is covered by rejecting 100000 in Decimal(5, 0) (which fits the 32-bit storage width) while accepting 99999, rejecting 1000.00 in Decimal(5, 2) while accepting 999.99 (the bound applies to the unscaled integer at the column scale), and confirming the check runs on the truncated value so 999.999 in Decimal(5, 2) still succeeds.
• ClickHouseIOIT — round-trip integration tests against the ClickHouse test container for Decimal(9, 2), Decimal(18, 4), Decimal(38, 10) and Decimal(76, 20) (including negative values), Nullable(Decimal(10, 2)) with a null row, Array(Decimal(9, 2)), a DEFAULT 2.25 column exercised end-to-end by writing a null row, a getTableSchema round-trip over a table declared with the width aliases (verifying we parse the server's canonicalized DESCRIBE output), and the user-visible truncation of over-scaled values (-1.239 → -1.23, confirming ClickHouse truncates toward zero rather than rounding, in agreement with the client-side encoding).

fixes #39840

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